The research described in this proposal involves studies of the structure of ligands near metal ions, electron spin relaxation times, and metal-metal separations in a series of biologically important metal complexes. Ligand structures will be investigated using low temperature nmr and endor spectroscopy. The internal dipolar field at ligand nuclei which is generated by unpaired electrons of the metal ions produces large shifts of nmr and endor signals. Analysis of the nmr and endor spectra allows one to determine metal-nuclei separations and the local geometry of nuclei surrounding the metal ion. Structural studies of this type will be conducted with some model systems and with a series of metal containing proteins, hemoglobins, and nucleic acids. A new technique has been developed to measure electron spin relaxation times of transition metal complexes. These relaxation times depend on the electronic structure of the metal complexes and dipolar and exchange interactions between the spins on different metal atoms. Relaxation times will be measured from a series of complexes to obtain information on crystal field splittings and metal-metal separations. The combined study of ligand structure, crystal field splitting, and metal-metal separations should produce valuable new information on the structure of these metal complexes. Structural information of this type is necessary for an understanding of the mechanism by which these metal complexes function in a variety of biological processes.